Hi Mitra,
In the U.S., a Bachelor's degree and an undergraduate degree are the same thing - they are 4-year college (university) degrees. The word "undergraduate" can also refer to the school or the student, while "Bachelor" only refers to the degree. "Graduate" refers to anything coming after a Bachelor's degree. It can be a Master's degree or a Doctorate (which is the highest degree offered). And again, you can also say "graduate student," "graduate school," and "graduate studies," in addition to "graduate degree."